Node Level Vectoring Synchronization

ABSTRACT

An apparatus for synchronizing a plurality of digital subscriber line access multiplexers (DSLAMs) comprising a reference clock configured to generate a reference clock signal, and an interface configured to transmit the reference clock signal to the DSLAMs, wherein each of the DSLAMs is configured to align its system clock with the reference clock.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Crosstalk may be a significant source of noise in wireline communicationsystems, especially those based on Digital Subscriber Line (DSL)technology. In International Telecommunication Union (ITU) standardG.993.5-2010 (i.e., G.vector), crosstalk may be mitigated by aligning orsynchronizing transmissions from a given DSL access multiplexer (DSLAM)(i.e., intra-DSLAM crosstalk). However, crosstalk among different DSLAMs(i.e., inter-DSLAM crosstalk) may not be addressed by the G.vectorstandard. Therefore, inter-DSLAM crosstalk may be a significant sourceof noise in wireline systems, especially those based on the G.vectorstandard.

SUMMARY

In a first aspect, the invention includes an apparatus for synchronizinga plurality of DSLAMs comprising a reference clock configured togenerate a reference clock signal, and an interface configured totransmit the reference clock signal to the DSLAMs, wherein each of theDSLAMs is configured to lock its system clock with the reference clock.

In another aspect, the invention includes a method for synchronizing aplurality of DSLAMs comprising generating a reference clock signal, andtransmitting the reference clock signal to the DSLAMs, wherein each ofthe DSLAMs is configured to align its system clock with the referenceclock.

In yet another aspect, the invention includes a DSLAM comprising aninterface that is configured to receive a trigger clock pulse from anode vector control entity (NVCE), at least one memory; and a processorconfigured to receive the trigger clock pulse, upon receiving thetrigger clock pulse, save values of a tick counter, a symbol counter,and a Sync symbol counter to the at least one memory, transmit thevalues of the tick counter, the symbol counter, and the Sync symbolcounter to the NVCE, receive a value of a first offset and a secondoffset, wherein the first offset is based on an offset between the tickcounter and an NVCE tick counter, and wherein the second offset is basedon the symbol counter, the Sync symbol counter, a NVCE symbol counter,and a NVCE Sync symbol counter, adjust the symbol boundary, the symbolcounter, and the Sync symbol counter based on the first offset and thesecond offset.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is an embodiment of a system for synchronizing DSLAMs.

FIG. 2 is another embodiment of a system for synchronizing DSLAMs.

FIG. 3 illustrates clock signals for a NVCE and a DSLAM.

FIG. 4 illustrates an embodiment of a method for powering up a NVCE anda DSLAM.

FIG. 5 illustrates a NVCE trigger clock signal.

FIG. 6 is a second illustration of a NVCE trigger clock signal.

FIG. 7 illustrates trigger clock marker pulses used to mark Sync symbolcounter values.

FIG. 8 illustrates a schematic diagram of an embodiment of ageneral-purpose network component.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Each of a plurality of DSLAMs may comprise a system vector controlentity (SVCE) module, which may perform centralized control for itscorresponding DSLAM, and may further comprise multi line-cards. Eachline-card may contain multiple lines. A SVCE module may control aninitialization of state flows, a cancellation vector engine, andsynchronization of DSL lines among line-cards within a DSLAM. A DSLAMmay be in synchronous mode, in which case DSL lines may synchronize bylocking the sampling clocks, aligning symbol boundaries, and aligningthe position of Sync symbols. If a cable binder is shared by multipleDSLAMs, it may be desirable for DSLAMs to be synchronized to haveeffective crosstalk cancellation.

Intra-DSLAM crosstalk has previously been addressed, but inter-DSLAMcrosstalk has not. For example, European Patent Application Publication1936825, entitled “A transient crosstalk controlling device,” by J.Sylvia Verlinden, relates to mitigating transient crosstalk between DSLloops extending from a node or a single line termination board(linecard) in an access node. Transient crosstalk may be noise inducedby a subscriber line into other subscriber lines before it can becancelled by a crosstalk cancellation mechanism. As another example,European Patent Application Publication 2023500, entitled “A device andassociated method for measuring crosstalk,” by M. Guenach, et al.,relates to a device comprising for transmitting a signal on the inactivesubscriber line and synchronously measuring crosstalk induced into theactive subscriber lines by transmitting the signal on the inactivesubscriber line. Synchronization among DSLAMs has not been addressedpreviously.

Systems and methods are presented herein that relate generally tosynchronization among DSLAMs that may assist with mitigating transientcrosstalk between DSL loops that may be shared in a same cable binder.Synchronization may be performed by locking a sampling clock, aligning asymbol boundary, and aligning synchronization symbols.

FIG. 1 is an embodiment of a system 100 for synchronizing DSLAMs. Thesystem 100 comprises a network reference clock (NTR) 110, a node vectorcontrol entity (NVCE) 120, and DSLAMs 130, 140, and 150 configured asshown in FIG. 1. Each of the DSLAMs 130, 140, and 150 may be coupled toone or more customer-premises equipments (CPEs), exemplary ones of whichare labeled as 162, 164, 166, and 168. An example CPE may be a DSLmodem. Each DSLAM may comprise a network terminal (NT), a SVCE, and oneor more line terminals (LTs), exemplary ones of which are labeled as132, 134, 136, and 138, respectively, and configured as shown in FIG. 1.Three DSLAMs 130, 140, and 150 are shown in FIG. 1 for illustrativepurposes, but the system 100 may comprise any number of DSLAMs. A CPE,such as CPEs 162, 164, 166, and 168, may be located in a customer'spremises, such as a home or business, whereas a DSLAM, such as DSLAMs130, 140, and 150, may be located in an operator's premises, such as atelephone exchange or a central office. A connection between a CPE and aDSLAM may be an electrical conductor, such as a twisted-pair copperline, as an example.

A NT (e.g., NT 132) may provide an interface between a DSLAM and othernetwork equipment on the operator side of a network. A SVCE (e.g., SVCE134) may provide centralized control for a DSLAM (e.g., 130) so that allCPE lines connected to the DSLAM may be synchronized. However, an SVCEmay not provide synchronization between DSLAMs.

The NTR 110 may provide a clock signal that may be used by all DSLAMs130, 140, and 150. The NVCE 120 may generate a common reference clockand input to all the DSLAMs 130, 140, and 150. The common referenceclock may be synchronized with the NTR 110 clock. Each DSLAM 130, 140,150 may lock its system clock and sampling clock with the commonreference clock. In order to synchronize a sampling clock of all CPElines, each DSLAM may have an input pin accepting either NTR clock or acommon reference clock from the NVCE 120. Information between DSLAMs andCPEs may be exchanged via multitone modulation schemes, such as discretemultitone (DMT) modulation or orthogonal frequency division multiplex(OFDM) modulation. For convenience, the discussion focuses on DMTmodulation, but the systems and methods described herein also apply toOFDM modulation.

FIG. 2 is another embodiment of a system 200 for synchronizing DSLAMs.The system 200 comprises a NTR 210, a NVCE 220, and DSLAMs 230, 240, and250 configured as shown in FIG. 2. The NVCE 220 comprises a phase lockloop (PLL) 222 and a clock/pulse generator module 224 configured asshown in FIG. 2.

A local clock (not shown) in NVCE 220, which generates local clocksignal (Sclk), may be synchronized with the NTR clock via the PLL 222.An output of the PLL 222 may be fed into the clock/pulse generatormodule 224. The clock/pulse generator module 224, which may also bereferred to as a reference clock, may output two signals, commonreference clock signal (Ref_clk) and trigger clock signal (T_clk). Otherclocks in clock/pulse generator module 224 may be synchronous 4kilohertz (kHz) and 70.656 megahertz (MHz) clocks. The reference clocksignal Ref_clk may be connected to all the DSLAMs. The reference clocksignal Ref_clk may be transmitted to all the DSLAMs through standardnetwork interface hardware in the NVCE 220.

Each DSLAM may comprise a PLL. In each DSLAM, a local system clock maybe locked with Ref_clk through the DSLAM's PLL. Each DSLAM may alsogenerate synchronous 4 KHz and 70.656 MHz clocks. A DMT symbol may begenerated in a DSLAM based on its 4 KHz clock. The 4 KHz clocks in NVCE220 and a DSLAM may not be aligned. Each DMT symbol may have a durationof 1/4 KHz=250 microseconds (usec). A tick counter may use a 70.656 MHzclock, as an example, but the tick counter may be much faster than70.656 MHz to get a better resolution. A clock ratio may be 70.656 MHz/4KHz=17664 ticks per DMT symbol, which may equate to 250usec/17664=14.153 nanoseconds (nsec)/tick. DMT symbols may be numberedfrom symbol 0 to symbol 256, with DMT symbol 256 designated as a Syncsymbol, also known as a synchronization symbol (i.e., every 257^(th) DMTsymbol may be a Sync symbol). In general, DMT symbols may be numberedfrom symbol 0 to symbol M, where M is a positive integer, with symbol Mdesignated as a Sync symbol.

The NVCE 220 may comprise several counters—a NVCE symbol counter (SCnt),a NVCE Sync symbol counter (SSCnt), and a NVCE tick counter (TCnt). AnNVCE symbol counter (SCnt) repeatedly counts from DMT symbol 0 to symbol256. The NVCE symbol counter may be implemented in hardware or softwareand may be based on a 4 KHz clock in the NVCE 220. The Sync symbolcounter may be incremented by one for every 257 DMT symbols. The Syncsymbol counter may be modulo of 1024. The Sync symbol counter may beimplemented in hardware or software. The NVCE tick counter repeatedlycounts from a first DMT symbol (i.e., symbol counter=0) sample to a lastDMT symbol (i.e., symbol counter=256) sample. The tick counter may beimplemented in hardware. The tick counter may be modulo by257*17664=4539648 ticks.

A pilot sequence length may be 2^(n), where n is an integer value. Aminimum pilot sequence length may be greater than a number of subscriberlines, and the maximum pilot sequence length may be 512. The pilotsequence may be an orthogonal sequence, such as a Hadamard binarysequence. Each DSL subscriber line may use a different pilot sequence ormay use the same pilot sequence. Pilot sequences may be applied to Syncsymbols such that the pilot sequence counter has reached the length ofthe pilot sequence and the process repeats. That is, the pilot sequencecounter=mod[Sync symbol counter, pilot sequence length], where “mod”denotes the modulo operator. By synchronizing the Sync symbol countersin a DSLAM with the NVCE Sync symbol counter, the pilot sequence may besynchronized.

Synchronization between DSLAMs 230, 240, and 250 may occur as follows.For illustrative purposes, synchronization of one DSLAM (e.g., DSLAM230, 240, or 250) with NVCE (e.g., NVCE 220) clocks is discussed, butsynchronization of each DSLAM with NVCE 220 clocks may be similar.Synchronization is explained with the assistance of FIG. 3, whichillustrates clock signals for a NVCE and a DSLAM. In particular, FIG. 3illustrates a NVCE tick counter clock signal 310, a NVCE symbol clocksignal 320, a trigger clock signal 330, a DSLAM tick counter clocksignal 340, and a DSLAM symbol clock signal 350. A NVCE and a DSLAM maybe considered to be in a master-slave relationship.

NVCE may send a pulse via trigger clock 330 (labeled as step 1). On theraising edge of T_clk NVCE may save values of tick counter (TCnt_M),symbol counter (SCnt_M), and Sync symbol counter (SSCnt_M) into one ormore memories (labeled as step 2). A DSLAM may detect a rising edge ofT_clk, and a DSLAM may have an input pin for accepting trigger clocksignal T_clk from a NVCE. Upon the rising edge of T_clk, a DSLAM maysave its value of tick counter (TCnt_S), symbol counter (SCnt_S), andSync symbol counter (SSCnt_S) into one or more memories. (Each DSLAM mayhave its own tick counter, symbol counter, and Sync symbol counter.)Next NVCE may obtain values TCnt_S, SCnt_S, and SSCnt_S from the DSLAM(labeled as step 3). NVCE may obtain these values by receiving them fromthe DSLAM or by reading the values from one or more memories in theDSLAMs, NVCE then computes an offset between tick counters asOffset_tick_ref=TCnt_M−TCnt_S and an offset in DMT symbols asOffset_symbol_ref=(SCnt_M+257*SSCnt_M)−(SCnt_S+257*SSCnt_S). Next NVCEsends values Offset_tick_ref and Offset_symbol_ref to the DSLAM (labeledas step 4). Finally, the corresponding DSLAM computessample_offset=mod[Offset_tick_ref, 17664],SCnt_new=mod[SCnt_S+symbol_elapsed+Offset_symbol_ref, 257], andSSCnt_new=mod[SCnt_S+symbol_elapsed+Offset_symbol_ref, 257*1024] andadjusts its symbol boundary by sample_offset ticks, its symbol counterto SCnt_new, and its sync symbol counter to SSCnt_new (labeled as step5), where duration “symbol elapsed” in FIG. 3 may be a duration in DMTsymbols between a rising edge of T_clk and a time when the DSLAM adjustsits counters.

After following the procedure described above, the 4 kHz clocks of NVCEand DSLAM may be synchronized. If multiple DLSAMs follow this procedure,the DSLAMs may be synchronized. Synchronization of the 4 kHz clocks maybe finalized by delaying/advancing the 4 kHz clock by sample_offsetticks in the DSLAM. Another way to align the symbol boundary withoutmodifying the DSLAM 4 kHz clock may be either delaying or advancing thetransmit/receive hardware buffer pointers. The DSLAM may adjust ahardware buffer pointer by a number of samples, Num_sample=round[T_sampling/(sample_offset*14.153 nsec)], where T_sampling is a DSLAMsampling clock. Another embodiment to align the symbol boundaries may befor a DSLAM to lock its 4 kHz clock with a NVCE 4 kHz clock by using aPLL in the DSLAM to phase lock the clocks. If the DSLAM 4 kHz clock isphase locked, a symbol boundary may be aligned at 4 KHz boundary.

FIG. 4 illustrates an embodiment of a method 400 for powering up a NVCEand a DSLAM. The NVCE may be powered on first in step 410, and maygenerate 4 kHz, 70.656 MHz, and Ref_clk clock signals. A DSLAM may bepowered on and/or booted up next in step 412. In step 414, a DSLAM clockmay be synchronized with Ref_clk via a PLL, and a DSLAM may generate 4kHz and 70.656 MHz clocks. In step 416, the NVCE and DSLAM maycommunicate with each other to verify that they are on, and DSLAM maysend system parameters, such as the clock frequency of tick counter(e.g., 70.656 MHz), to the NVCE. In step 418, the NVCE may send a pulsevia a trigger clock. In step 420, the DSLAM may send parameters, such astick counter (TCnt_S), symbol counter (SCnt_S), and Sync symbol counter(SSCnt_S) to the NVCE. In step 422, the NVCE may compute and send anoffset between tick counters (Offset_tick_ref) and may compute and sendan offset between DMT symbols (Offset_symbol_ref) to the DSLAM. In step424, the DSLAM may adjust its symbol boundary, symbol counter, and Syncsymbol counter based on information received from the NVCE. The DSLAMand NVCE may be synchronized after step 424.

In step 426, a determination may be made whether synchronization wassuccessful. If not, the method repeats the synchronization process insteps 416-426. If so, the method 400 continues at step 428, in which theDSLAM may be activated. In step 430, the DSLAM may start theinitialization procedure and the NVCE may control the state flow of theinitialization procedure. Finally, in step 432, the DSLAM and NVCE mayenter the linked state (e.g., referred to as “Showtime” in an xdslstandard) and steady data transmission may commence. At various timeintervals, a check may be performed in step 434 whetherresynchronization may be needed. (For example, if CPEs experience linedropping or insufficient performance, it may be beneficial for DSLAMs toresynchronize.) If so, the steps of the synchronization process 416-426may be repeated. If not, the process continues at step 428.

FIG. 5 illustrates a NVCE trigger clock signal (T_clk). The triggerclock sends a pulse every Sync symbol (e.g., Sync symbols 0 and 1 arelabeled as SS#0 and SS#1, and values of Sync symbol counter may be 0 and1 corresponding to Sync symbols 0 and 1). The T_clk rising edges may bealigned with the 4 kHz clock, and the DMT symbol boundary may also bealigned with T_clk. A DSLAM may detect the rising edge of T_clk toindicate the start of DMT symbol 256. A duration of a symbol may be2N+CE, where 2N is a size of an inverse fast Fourier transform (IFFT)used to construct a symbol and CE is a length of a cyclic extension,which may comprise a cyclic prefix and/or cyclic suffix.

FIG. 6 is a second illustration of a NVCE trigger clock signal (T_clk).Also illustrated are corresponding Sync symbol counter values SS#0through SS#1023. A Sync symbol counter may repeat every 1024 Syncsymbols. The rising edge of T_clk may correspond to a symbol boundary,where symbol count equals 256.

One drawback of previously discussed methods for synchronization may bethat a DSLAM may not know the Sync symbol counter value. FIG. 7illustrates trigger clock marker pulses used to mark Sync symbol countervalues. Marker pulses may be used in order to synchronize Sync symbolcounters between a NVCE and a DSLAM. At Sync symbol counter 0 (SS#0),the trigger clock may send a normal pulse. At Sync symbol counter 255(SS#255), the trigger clock may send two short pulses within a normalpulse time period. At Sync symbol counter 511 (SS#511), the triggerclock may send three short pulses. Finally, at Sync symbol counter 767(SS#767), the trigger clock may send four short pulses. At Sync symbolcounter 0, the trigger clock starts over with one pulse. Marker pulsesmay be pulse width modulated.

An advantage of marker pulses is that DSLAMs may not be required to feedback information to an NVCE in order for the DSLAMs to becomesynchronized. Marker pulses may be implemented in a system with DSLAMs,such as system 200 in FIG. 2.

Various systems and methods described above may be implemented on ageneral-purpose network component, such as a computer or networkcomponent with sufficient processing power, memory resources, andnetwork throughput capability to handle the necessary workload placedupon it. FIG. 8 illustrates a schematic diagram of an embodiment of ageneral-purpose network component 800. Specifically, the general-purposenetwork component 800 may be suitable for implementing one or moreembodiments of the methods or components disclosed herein, such as NTRs(e.g., NTR 210), NVCEs (e.g., NVCE 220), DSLAMs (e.g., DSLAM 230), CPEs(e.g., CPE 162), or portions thereof. The network component 800 includesa processor 802 (which may be referred to as a central processor unit orCPU) that is in communication with memory devices including secondarystorage 804, read only memory (ROM) 806, random-access memory (RAM) 808,input/output (I/O) devices 810, and network connectivity devices 812.The processor 802 may be implemented as one or more CPU chips, or may bepart of one or more application specific integrated circuits (ASICs).The I/O devices may, for example, comprise an interface between an NVCE(e.g., NVCE 220) and DSLAMs (e.g., DSLAM 230).

The secondary storage 804 is typically comprised of one or more diskdrives or tape drives and is used for non-volatile storage of data andas an over-flow data storage device if RAM 808 is not large enough tohold all working data. Secondary storage 804 may be used to storeprograms that are loaded into RAM 808 when such programs are selectedfor execution. The ROM 806 is used to store instructions and perhapsdata that are read during program execution. ROM 806 is a non-volatilememory device that typically has a small memory capacity relative to thelarger memory capacity of secondary storage 804. The RAM 808 is used tostore volatile data and perhaps to store instructions. Access to bothROM 806 and RAM 808 is typically faster than to secondary storage 804.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R₁, and an upper limit,R_(u), is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R₁+k*(R_(u)−R₁), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. Use of the term “optionally” with respect to anyelement of a claim means that the element is required, or alternatively,the element is not required, both alternatives being within the scope ofthe claim. Use of broader terms such as comprises, includes, and havingmay be understood to provide support for narrower terms such asconsisting of, consisting essentially of, and comprised substantiallyof. Accordingly, the scope of protection is not limited by thedescription set out above but is defined by the claims that follow, thatscope including all equivalents of the subject matter of the claims.Each and every claim is incorporated as further disclosure into thespecification and the claims are embodiment(s) of the presentdisclosure. The discussion of a reference in the disclosure is not anadmission that it is prior art, especially any reference that has apublication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it may be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus for synchronizing a plurality ofdigital subscriber line access multiplexers (DSLAMs) comprising: areference clock configured to generate a reference clock signal; and aninterface configured to transmit the reference clock signal to theDSLAMs, wherein each of the DSLAMs is configured to align its systemclock with the reference clock.
 2. The apparatus of claim 1, furthercomprising: a trigger clock configured to generate a trigger clockpulse; a tick counter clock; at least one memory; and a processorconfigured to: upon a rising edge of the trigger clock pulse, savevalues of a tick counter, a symbol counter, and a Sync symbol counterinto the at least one memory, wherein the tick counter, the symbolcounter, and the Sync symbol counter are based on a count of clockcycles of the tick counter clock; receive values of a DSLAM tickcounter, a DSLAM symbol counter, and DSLAM Sync symbol counter from eachof the at least one DSLAMs; for each of the DSLAMs, compute a firstoffset between the tick counter and the DSLAM tick counter, and computea second offset in symbols based on the values of the symbol counter,the Sync symbol counter, the DSLAM symbol counter, and the DSLAM Syncsymbol counter, wherein, for each of the DSLAMs, the apparatus isconfigured to transmit the first offset and the second offset to thecorresponding DSLAM so that the corresponding DSLAM can synchronize withthe apparatus.
 3. The apparatus of claim 2, wherein synchronizing aDSLAM with the apparatus comprises synchronizing the DSLAM symbolboundary, symbol counter, and Sync symbol counter with the apparatussymbol boundary, symbol counter, and Sync symbol counter.
 4. Theapparatus of claim 3, wherein the apparatus further comprises a localclock and a phase locked loop (PLL), wherein the PLL is coupled to thereference clock, wherein the PLL is configured to receive a clock signalfrom a network reference clock and lock the local clock with the networkreference clock.
 5. The apparatus of claim 4, wherein the DSLAMs employmultitone modulation with a Sync symbol inserted every M symbols, whereM is an integer, and wherein, for each of the DSLAMs, the second offsetequals (symbol counter+M*Sync symbol counter)−(DSLAM symbolcounter+M*DSLAM Sync symbol counter).
 6. The apparatus of claim 5,wherein a pilot sequence counter of at least one of the DSLAMs isdetermined based on the Sync symbol counter.
 7. The apparatus of claim6, wherein the DSLAMs share a cable binder, and at least one of theDSLAMs is configured to mitigate crosstalk from at least one of theother DSLAMs.
 8. A method for synchronizing a plurality of digitalsubscriber line access multiplexers (DSLAMs) comprising: generating areference clock signal; and transmitting the reference clock signal tothe DSLAMs, wherein each of the DSLAMs is configured to align its systemclock with the reference clock.
 9. The method of claim 8 furthercomprising: transmitting a trigger clock pulse to the DSLAMs; upontransmitting the trigger clock pulse, saving values of a tick counter, asymbol counter, and a Sync symbol counter into at least one memory in anode vector control entity (NVCE); receive values of a DSLAM tickcounter, a DSLAM symbol counter, and DSLAM Sync symbol counter from eachof the DSLAMs; for each of the DSLAMs, computing a first offset betweenthe tick counter and the DSLAM tick counter, computing a second offsetin symbols based on the values of the symbol counter, the Sync symbolcounter, the DSLAM symbol counter, and the DSLAM Sync symbol counter,and transmitting the first offset and the second offset to thecorresponding DSLAM; transmitting the first offset and the second offsetto the DSLAMs so that the DSLAMs synchronize with the NVCE.
 10. Themethod of claim 9, wherein a DSLAM synchronizing with the NVCE comprisessynchronizing the DSLAM symbol boundary, symbol counter, and Sync symbolcounter with the symbol boundary, symbol counter, and Sync symbolcounter.
 11. The method of claim 10, wherein the NVCE further comprisesa system clock, wherein the NVCE is configured to receive a clock signalfrom a network reference clock, and wherein the reference clock is basedon locking the system clock with the network reference clock.
 12. Themethod of claim 11, wherein the DSLAMs employ multitone modulation witha Sync symbol inserted every M symbols, where M is an integer.
 13. Themethod of claim 12, wherein a pilot sequence counter of at least one ofthe DSLAMs is determined based on the Sync symbol counter.
 14. Themethod of claim 13, wherein the DSLAMs share a cable binder, and whereinat least one of the DSLAMs is configured to mitigate crosstalk from atleast one of the other DSLAMs.
 15. A digital subscriber line accessmultiplexer (DSLAM) comprising: an interface that is configured toreceive a trigger clock pulse from a node vector control entity (NVCE):at least one memory; and a processor configured to: receive the triggerclock pulse; upon receiving the trigger clock pulse, save values of atick counter, a symbol counter, and a Sync symbol counter to the atleast one memory; transmit the values of the tick counter, the symbolcounter, and the Sync symbol counter to the NVCE; receive a value of afirst offset and a second offset, wherein the first offset is based onan offset between the tick counter and an NVCE tick counter, and whereinthe second offset is based on the symbol counter, the Sync symbolcounter, a NVCE symbol counter, and a NVCE Sync symbol counter; adjustthe symbol boundary, the symbol counter, and the Sync symbol counterbased on the first offset and the second offset.
 16. The DSLAM of claim15, wherein the interface is further configured to receive a referenceclock signal from the NVCE, the DSLAM further comprising: a tick counterclock; a system clock; and a phase lock loop configured to synchronizethe system clock with the reference clock signal, wherein the tickcounter, the symbol counter, and the Sync symbol counter are based on acount of clock cycles of the tick counter clock, and wherein the tickcounter, the symbol counter, and the Sync symbol counter are referencedrelative to a clock cycle boundary of the system clock.
 17. The DSLAM ofclaim 16, wherein the DSLAM employs multitone modulation with a Syncsymbol inserted every M symbols, where M is an integer, wherein thefirst offset equals the difference between the NVCE tick counter and thetick counter, and wherein the second offset equals (NVCE symbolcounter+M*NVCE Sync symbol counter)−(symbol counter+M*Sync symbolcounter).
 18. The DSLAM of claim 17, wherein a pilot sequence counter ofthe DSLAM is determined based on the Sync symbol counter.
 19. The DSLAMof claim 18, wherein the DSLAM shares a cable binder with a secondDSLAM, and wherein the DSLAM is configured to mitigate crosstalk fromthe second DSLAM.
 20. The DSLAM of claim 19, wherein the NVCE and theDSLAM are in a master-slave relationship, respectively.